1. Field of the Invention
This invention relates to an optical wavelength converter element for converting a fundamental wave to a second harmonic wave, and more particularly to a method for forming domain reversals in a predetermined pattern on a ferroelectric possessing the nonlinear optical effect, in order to fabricate an optical wavelength converter element having periodic domain reversals.
2. Description of the Prior Art
There has already been proposed by Bleombergen et al. in Physics Review vol. 127, No. 6 in 1918 (1962), in which the wavelength of a fundamental wave is converted into a second harmonic wave using an optical wavelength converter element with regions (domains) where the directions of spontaneous polarization of a ferroelectric, possessing the non-linear optical effect, are periodically switched.
In this method, the fundamental wave can be phase-matched with the second harmonic wave by setting the period A of the domain reversals to be an integral multiple of the coherent length .LAMBDA.c which is given by EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
where .beta.(2.omega.) designates the propagation constant of the second harmonic wave, and 2.beta.(.omega.) represents the propagation constant of the fundamental wave. When wavelength conversions are effected using the bulk crystal of a nonlinear optical material, a wavelength to be phase-matched is limited to the specific wavelength inherent to the crystal. However, in accordance with the above-described method, a phase matching can be realized effectively by selecting a period .LAMBDA. satisfying the condition (1) for an arbitrary wavelength.
Examples of the known fabrication method for such a periodic domain reversals include
1) the method proposed by K. Yamamoto, K. Mizuuchi, and T. Taniuchi in Optics Letters. Vol. 16, No. 15, pp. 1156 (1991) wherein the -z surface of LiTaO.sub.3 is periodically subjected to proton-exchanges, and a resultant structure undergoes a heat treatment around the Curie temperature; and PA1 2) the method proposed by H. Ito, C. Takyu, and H. Inaba in Electronics Letters, vol. 27, No. 14, pp. 1221 (1991), wherein electron beams are directly radiated onto the -z surface of LiTaO.sub.3 or LiNbO.sub.3 at room temperature. PA1 forming a proton-exchanged region in a predetermined pattern on a ferroelectric which possesses the unipolarized nonlinear optical effect; and PA1 heating the proton-exchanged region with the application of an electric field from the outside, thereby fabricating local domain reversals. PA1 forming a Ti-diffused region in a predetermined pattern on a ferroelectric which possesses the unipolarized nonlinear optical effect; and PA1 heating the Ti-diffused region with the application of an electric field from the outside, thereby fabricating local domain reversals. PA1 forming an outer diffused region in a predetermined pattern on a ferroelectric possessing the unipolarized nonlinear optical effect; and PA1 heating the outer diffused region with the application of an electric field from the outside, thereby fabricating local domain reversals. PA1 implanting ions or atoms in a predetermined pattern into a ferroelectric, from one end surface thereof, which possesses the unipolarized nonlinear optical effect, so that the electric conductivity of the implanted region is changed; and PA1 locally inverting the directions of polarization at the implanted region or the remaining region by applying an electric field to the intermediate region sandwiched between the implanted surface and the other opposite end surface. PA1 irradiating light, at the wavelength enough to induce the photo refractive effect, in a predetermined pattern over a ferroelectric which possesses the unipolarized nonlinear optical effect and the photorefractive effect; and PA1 transforming the region, where the photorefractive effect appears, into local domain reversals by applying an electric field from the outside. PA1 forming an electrode in a predetermined pattern on one surface of a ferroelectric which possesses the unipolarized nonlinear optical effect; PA1 subjecting the ferroelectric to a corona electrical charging by means of the electrode and a corona wire disposed on the other opposite surface side of the ferroelectric; PA1 applying an electric charge to the region having undergone the corona electrical charging; and PA1 transforming the region occupied by the electrode of the ferroelectric into local domain reversals. PA1 forming an electrode in a predetermined pattern on one surface of a ferroelectric which possesses the unipolarized nonlinear optical effect; PA1 rapidly cooling the ferroelectric after the ferroelectric has been heated with the electrode thereof grounded; and PA1 transforming the region occupied by the electrode of the ferroelectric into local domain reversals by applying an electric field to the ferroelectric utilizing surface electric charges resulting from the pyroelectric effect. PA1 an electric field is applied to the ferroelectric in dry atmosphere or under vacuum.
In connection with the practice of the first method, there is also proposed that a waveguide type optical wavelength converter element be fabricated by forming, employing proton exchanges, channel waveguides on preliminarily prepared third-order periodic domain reversals. When this optical wavelength converter element is observed in cross section, it turns out that hemicyclic periodical domain reversals are formed. When a Ti:Al.sub.2 O.sub.3 laser is used as the light source of a fundamental wave, there is outputted a second harmonic wave of 2.4 mW in response to a fundamental wave input of 99 mW, thereby achieving a wavelength conversion efficiency approximate to theoretical values at the third-order period.
In the meantime, the optical wavelength converter element, which is fabricated in accordance with the second method, is provided with periodic domain reversals extending, from one end to the other, in the thicknesswise direction of the LiNbO.sub.3 substrate (e.g. about 0.5 mm in width), that is, extending through the substrate from the -z surface to the +z surface. Thus, this type of element can find applications as a bulk type wavelength converter element. In the optical wavelength converter element with the third-order periodic domain reversals, fabricated in accordance with the above technique, phase matching is observed in the bulk by the wavelength sweep of a Ti:Al.sub.2 O.sub.3 laser.
In the case of the waveguide type optical wavelength converter element, in order to obtain a second harmonic wave at a practical level of several milliwatts using a semiconductor laser of a single lateral mode which has an output of about 100 mW as a fundamental wave light source, it is necessary to attain an improved wavelength conversion efficiency which is about one order higher than the third-order periodic structure by forming first-order periodic domain reversals.
In the aforementioned first method, since the domains have been reversed by a space-charge electric field developed in proton-exchanged regions during the heat treatment, the depth of the domain reversal is as shallow as about 1.6 .mu.m at a first-order micro period of 3.6 .mu.m. As a result of this, the domain reversals are formed considerably less deep than the thickness of a waveguide, which is generally of about 2.4 .mu.m, thereby hindering the increase of the overlap integral between the fundamental wave and the domain reversals, and eventually achieving only the efficiency which is nearly twice that of the third-order periodic structure. Another drawback of the first method lies in that the proton-exchanged regions will probably be dispersed unless the optical wavelength converter element is immediately heated up to a temperature of about 500.degree.-600.degree. C., that is the temperature of the heat treatment. This makes the first method difficult with respect to reproducibility.
Contrary to this, the second method enables domain reversals to be manufactured deeply enough to reach the rear end of the substrate, but this method is admittedly inferior in the control of the domain reversals. Due to the capacity of an electron beam annealing system, the area that is exposed to radiations at one time is restricted to an area measuring about 2.times.2 mm. In order to improve the wavelength conversion efficiency by elongating the interactive length between the element and the fundamental wave, the periodic domain reversals should be interconnected with one another accurately, but the accuracy of the interconnection is admittedly difficult to ensure. This second method is also disadvantageous in that the use of the SEM, which requires a long time to scan across a wide area, hinders the improvement of the productivity of the elements.